1. Field of the Invention
The present invention relates to a valve for recirculating exhaust gas which enables harmful components such as NO.sub.x contained in exhaust gas discharged from an internal combustion engine to be kept as low as possible by re-circulating the exhaust gas discharged from the internal combustion engine through an intake system.
2. Description of the Related Art
Valves for re-circulating exhaust gas have been hitherto used in order to remove harmful components discharged from internal combustion engines. For example, U.S. Pat. No. 5,020,505 discloses a system having a mechanism in which an electromagnetic coil assembly 72 is used to displace a shaft 38 of a valve member 36, wherein the valve member 36 is separated from a seat portion 24, thereby permitting an inlet 20 to communicate with an outlet 22 (see FIG. 10). More specifically, in this mechanism, an armature core 76 is displaced by applying a predetermined voltage to the electromagnetic coil assembly 72, so that the valve member 36 integrally formed with the armature core is opened.
On the other hand, Japanese Laid-open Utility Model Publication No. 3-77876 also discloses a valve for recirculating exhaust gas of this type. Japanese Laid-open Utility Model Publication No. 3-77876 discloses the following technique. A coil spring 15 and a coil spring 9 are arranged so that their restorative forces act in opposite directions to one another. A plunger 14 is lifted while opposing to the restorative force of the coil spring 15 by applying a predetermined voltage to a coil 12. Concurrently, the restorative force of the coil spring 9 is used to move a valve shaft 6a toward the coil 12, so that a valve body 6 is separated from a valve seat 4. Thus, an inlet port 2 is allowed to communicate with an outlet port 3 (see FIG. 11).
In order to prevent any mismatch that may arise between the shaft centers of the armature core 76 and the valve member 36 in one conventional technique described in U.S. Pat. No. 5,020,505, and shown in FIG. 10, the armature core 76 and the valve member 36 are constructed so that a certain amount of deviation in directions perpendicular to an axial direction of the shafts is permitted. However, in such a system, when the valve member 36 is seated on the seat portion 24 on an outlet side, the combined inertial forces of the armature core 76 and the valve member 36 act on the seat portion 24. Accordingly, as the frequency of use increases, the valve member 36 and the seat portion 24 undergo abrasion and deformation. As a result, when the valve member 36 is seated on the seat portion 24, a bouncing motion or vibration takes place. Consequently, it has been difficult to achieve a reliable action for re-circulating exhaust gas with high accuracy.
In order to suppress the bouncing motion described above, it is possible to increase the restorative force (load) of a spring member so that the armature core 76 is energized in a direction simply to close the valve. However, such a countermeasure is accompanied by the necessity for an electromagnetic coil assembly for generating a large electromagnetic force to overcome the restorative force of the spring member. As a result, an inconvenience arises in that the electromagnetic coil assembly itself becomes large, and the production cost becomes expensive.
On the other hand, in another conventional technique described in Japanese Laid-open Utility Model Publication No. 3-77876, and shown in FIG. 11, the plunger 14, which is displaced by the coil spring 15, is structurally separated from the valve body 6 which actually works for opening and closing the valve. However, in such an arrangement, in addition to the coil spring 15 for seating the valve body 6 on the valve seat 4, a second coil spring 9 is required to allow the valve shaft 6a on one side of the valve body 6 to abut against a lower end of the plunger 14. Further, each of the coil springs 15, 9 have forces which act in directions that are opposite to one another, and the restorative force of the coil spring 15 must be selected to be greater than the restorative force of the coil spring 9. Therefore, the applied load for seating the valve body 6 on the valve seat 4 is a synthesized load based upon the respective loads of the two coil springs 15, 9. Namely, a load obtained by subtracting the load of the coil spring 9 for opening the valve from the load of the coil spring 15 for closing the valve is used for performing the action. This requires the necessity of an excessively large voltage to the coil 12 for opening the valve. Further, this technique includes the further inconvenience that, if the accuracy of the coil spring 15 for closing the valve and the accuracy of the coil spring 9 for opening the valve are different from each other, this difference directly and adversely affects the operational stability of the valve as it is controlled to be opened or closed, impairing the ability to perform control with high accuracy.